Pressure
Sensors
New Opportunities for
Gas Exchange Analysis
Using Piezoresistive
High-Temperature Absolute Pressure Sensors
Dr.-Ing. Andrea Bertola
Dipl.-Ing. Andreas Fürholz
Dipl.-Ing. Jürg Stadler
Dipl.-Ing. Jens Höwing
Kistler Instrumente AG,
Winterthur, Switzerland
Prof. Dr. Karl Huber
Dipl.-Ing. Johann Hauber
University of applied
sciences, Ingolstadt
Prof. Dr.-Ing.
Christoph Gossweiler
University of applied
sciences,
Northwestern Switzerland
Special Print
920-366e-10.08
Contents
1
Abstract......................................................................................................................................................................3
2
Motivation..................................................................................................................................................................3
3
Piezoresistive Sensors and Installation.......................................................................................................................6
3.1
4
Impact of the Sensor Position on the Measured Pressure...........................................................................................7
5
Characterisation of Measuring Accuracy and Influence on the Analytical Results......................................................9
5.1
Sensor Temperature Over the Engine Operating Range...................................................................................9
5.2
Accuracy Achieved by Using Piezoresistive Absolute Pressure Sensors.............................................................9
5.2.1 Pressure Measurement with Piezoresistive Sensors Direct Mounted/in a Cooling Adapter...............................9
5.2.2 Pressure Measurement with Piezoresistive Sensors Installed in a Cooled Switching Adapter..........................10
5.3
Temperature Characteristics of Piezoresistive Sensors.......................................................................................7
Low Pressure Indication with Piezoelectric Sensor and Pneumatic Pressure Measurement
(Remote Sensing System)..............................................................................................................................11
5.4
Influence of Absolute Pressure Level on the Result of the Gas Exchange Calculation.....................................13
6
Conclusion and Recommendations...........................................................................................................................14
7
References.................................................................................................................................................................14
Appendix: Applied Pressure Sensors and Cooling Adapters.....................................................................................15
2
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Pressure Sensors
New Opportunities for Gas Exchange Analysis Using Piezoresistive
High-Temperature Absolute Pressure Sensors
Dr.-Ing. Andrea Bertola, Dipl.-Ing. Andreas Fürholz, Dipl.-Ing. Jürg Stadler, Dipl.-Ing. Jens Höwing
Kistler Instrumente AG, Winterthur, Switzerland
Prof. Dr. Karl Huber, Dipl.-Ing. Johann Hauber, University of applied sciences, Ingolstadt
Prof. Dr.-Ing. Christoph Gossweiler, University of applied sciences Northwestern Switzerland
1 Abstract
The gas exchange influences to a large extent power, emissions and fuel consumption of internal combustion engines.
The analysis and optimization of the gas exchange is of
primary importance and will become more so with the new
homogeneous combustion utilizing high degrees of exhaust
gas recirculation (EGR).
Low pressure measurement using piezoresistive absolute
pressure sensors has become an important tool for the design
and optimization of the gas exchange, for the analysis of
process variables and for simulation validation. The advantages of an absolute pressure measurement are the high
precision (including dynamics) and the ability to resolve the
pressure differences between cylinders and single cycles.
Capturing both the dynamic behaviour and an exact pressure
level are critical for low pressure calculations, with new miniaturized piezoresistive pressure sensors installation is possible
directly into the cylinder head to enable this. A sensor position near the valve is most suitable as the effort for modelling
the system is reduced. In the exhaust manifold however, a
cooling adapter will be necessary unless the sensor is installed
directly in the cylinder head.
The utilization of a cooled switching adapter allows a precise
zero point correction of the piezoresistive pressure sensor,
therefore a reference precision (±1 mbar) can be achieved in
any operating condition.
Low pressure indication using a piezoelectric sensor and
pneumatic pressure measurement (remote sensing system)
is not recommended to evaluate the absolute pressure in the
exhaust.
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3
2 Motivation
Low pressure indication is the measurement of low amplitude
pressures plotted against the engine crank angle, typically
in the range of 0 … 5 bar absolute. The primary intention
of the low pressure indication is the dynamic measurement
of very small pressure changes within a few mbar. The particular challenge is that simultaneously the dynamic and the
absolute pressure level needs to be measured with high precision. Both of these important pressure characteristics form
the basic fundamental requirements for the simulation and
optimization of engines.
The main focus of the low pressure indication in engine
development is the analysis of the gas exchange (Fig. 1).
The direct potential for reducing fuel consumption and CO2
emissions is provided by minimizing the gas exchange losses.
This is done in gasoline engines for instance, by implementing
variable valve trains, downsizing and dethrottling in stratified
combustion concepts. In addition, the gas exchange plays an
important role in the reduction of pollutant emissions and
thus will contribute to help meet future emission regulations.
The gas exchange has become more and more an integral
part of the whole combustion strategy. New homogeneous
combustion concepts (CAI, HCCI), which combine the properties of gasoline and diesel engines, are distinguished by a
strong interaction between gas exchange and the subsequent
combustion [1]. These new combustion concepts can be
realized only with the controlled trapping of exhaust gases
during the gas exchange. As a result, the high dependence of
the combustion on the condition of the cylinder charge which
has been set during the gas exchange means that this control
has to be performed precisely.
The work done for the gas exchange, expressed as characteristic quantity PMEP, can be determined with today's
standard cylinder pressure indication systems. This value is
always available during the testing in the test bench, often
as real-time value. In addition to this global information, the
test engineer needs a more detailed insight as to the processes within the gas exchange. The most important parameter which influences the combustion is, besides the charge
motion, the residual gas fraction of the cylinder charge,
which influences the ignition, combustion behaviour and
combustion stability.
Gas exchange
Gas exchange losses
Reference measurement
Engine parts and systems
Combustion
chamber
1-D Model calibration
CFD
Combustion optimization
Heat release analysis
Residual gas control (HCCI)
Timing
Cam profile
Load control (cylinder charge)
Residual gas model
Intake manifold model
Fig. 1:
4
Low Pressure
Indication
Throttle &
actuator
Simulation
Intake/
Exhaust
Residual
gas
Special
tests
Crankcase
Pumps
Media
Charge motion
EGR system
Valves air/exhaust
Engine brake
Supercharging
systems
Exhaust gas
aftertreatment
Valves
Test bench
Acoustic
Functional
development
Applications utilizing low pressure indication
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The residual gas fraction can’t be measured directly, gas
exchange analysis be it 0-D or 1-D simulations, are necessary for this determination. Simple residual gas models can
be used for the fast calculation of the residual gas fraction
[2]. These models achieve precise results over wide operating ranges but without considering the dynamics. Cylinder
selective information as well as the interpretation of the
complicated gas dynamic behaviour, for instance, during the
valve overlapping time, can only be obtained by performing
detailed gas exchange analysis. The results permit the identification of variables and the customized optimization of the
single processes existing during the gas exchange.
The speed of the development process nowadays, requires
that the gas exchange analysis is performed at the test
bench; the results are used directly in the application of any
adjusted operating condition. Computed values, such as the
EGR rate are then saved as real data; e.g. pressures or temperatures with further test bench data.
Gas exchange analysis requires as an input, in addition to
cylinder pressure, the indicated pressures within intake and
exhaust systems. The choice of the measuring position for
the low pressure sensors is influenced mainly by the accessibility to the engine itself. The influence of the sensor position on the results of the gas exchange analysis depends on
the methods used for the computational analysis. 1-D gas
exchange analysis considers the running time of the pressure wave propagation between the sensor position and the
cylinder.
Previous works [3, 4] studied the influence of the sensor
measuring position on the results of 0-D gas exchange
analysis. For measuring positions close to the valve it was
determined that it is negligible for passenger car engine
speeds. In the same investigation the absolute pressure in
the intake and exhaust was identified as a key parameter for
the accuracy of the simulation results. The required precision
of the absolute indicated pressure in the intake and exhaust
was quantified as ±10 mbar.
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Low pressure indication is the central criteria for ensuring
the accuracy of gas exchange simulation calibrations. In 1-D
simulations, control points are modelled in accordance with
the measuring positions on the engine, there the indicated
pressures are compared with the computed pressure curves.
The low pressure indication is therefore the reference for the
dynamic behaviour and absolute pressure. Differences of the
computed pressures will be matched by varying the model
parameters e.g. lengths and discharge flow coefficients. The
quality of the low pressure indication is therefore a condition
for the final accuracy of the simulation [5].
According to the aim of the investigation and in view of the
special properties of the measurement technology, the measuring positions for the low pressure indication can be varied.
Space availability and the high temperature of the exhaust
are challenging aspects to the sensor dimensions and the
sensor adaptation in general.
High accuracy and process reliability are essential factors
for the general application of any low pressure indication
designated for the optimization of the gas exchange. The
development of new combustion concepts and the wide
use of many technologies demand from an Engineer, a
deep understanding of the processes governing the internal
combustion engine. Low pressure indication provides, in this
sense, important measured values with which further detailed
analysis and data processing is possible.
So the question arises:
Is low pressure indication, as a development tool, becoming a
standard measuring technology in engine development?
5
3 Piezoresistive Sensors and Installation
The maximum operating temperature of piezoresistive sensors for low pressure indication is often lower than that of the
measured media. Long term measurements are only possible
with special arrangements, such as water cooling the sensor
or with the use of a cooled switching adapter. Fig. 2 identifies a number of installation alternatives for piezoresistive
absolute pressure sensors in the intake and exhaust manifold
of an internal combustion engine.
The dimensions of the sensor are very important for direct
mounting into the cylinder head, for this, the compact hightemperature sensor Type 4007B is ideally suited. This application occurs mainly in the development of motor sport engines
where concerns for the size and mass of any additional
engine mounted hardware are most acute.
Pressure range, compensation technique and thermal properties of the Kistler pressure sensors are shown in Fig. 3.
d
a
b
DCE-Sensor
c
Thread size
Intake
a
b
c
d
e
Exhaust
Sensor Type 4005B
Sensor Type 4007B, direct installed in cyl. head
Sensor Type 4007B, cooling adapter Type 7525A
e
Sensor Type 4075A, cooling adapter Type 7505
Sensor Type 4045A/4075A/4007B,
cooled switching adapter Type 7533A
f Sensor Type 4045A, cooling adapter Type 7511
Fig. 2:
Type 4005B
Type 4007B
Type 4045A
Type 4075A
M5x0,5
M5x0,5
M14x1,25
M12x1
0 ... 10
Measuring
range
bar
0 ... 5/10
0 ... 5
0 ...
2/5/10
Max.
temperature
°C
125
200
140
140
Type of
compensation
–
Analog
Analog (+
digital with
amplifier
Type 4665)
Analog
Analog
Compensated
temperature
range
°C
0 ... 125
0 ... 180
20 ... 120
20 ... 120
Thermal zero
shift
%FSO
<1
<1
<0,5
<0,5
Thermal sensitivity shift
±%
<1
<1
<1
<1
f
Different applications of piezoresistive sensors in intake
and exhaust
Due to the limited availability of space and the geometry of a
modern intake manifold, size is the main requirement for the
sensor. The reduced dimensions of miniaturized piezoresistive pressure sensors fulfil this requirement and in particular,
Kistler Type 4005B/Type 4007B (M5) sensors are well suited
for direct installation into the intake manifold or the cylinder
head. In addition to such size considerations, temperatures of
up to 120 °C are possible within the intake, particularly with
high levels of EGR, however, the sensor is capable of withstanding these temperatures without additional cooling.
Another benefit of the small diameter (M5) is that the head
of the sensor can be seated flush with the inside surface of
the intake channel.
In the exhaust, higher temperatures (over 1 000 °C) require
active sensor cooling. This can be achieved by utilizing dedicated cooling adapters or the cooling of the cylinder head.
In the simple cooling adapter (Type 7525A, M14) the sensor
housing is cooled but the sensor diaphragm is exposed to the
hot gases. The cooled switching adapter (Type 7533A, M14)
employs a switching mechanism which is opened by a pneumatic valve during the time of the measurement only. This
maximises the sensor lifetime as well as making a correction
of the zero point possible while the engine is still running.
6
–
Oil filled Sensor
Fig. 3:
Specification of piezoresistive measuring chains
For low pressure indication with absolute piezoresistive sensors Kistler provides two types of construction. The Direct
Chip Exposed principle (DCE) is the new, miniaturized sensor generation Type 4005B/Type 4007B (M5) wherein the
semi-conductor measuring element is directly exposed to the
media and is coated with a special protective film. Oil filled
sensors, Type 4045A (M14) and Type 4075A (M12), utilize
a similar measuring principle which requires a slightly larger
package. In this design, the sensor has a thin steel isolation
diaphragm which provides a high resistance to soot and particle emissions.
All piezoresistive pressure sensors have thermal effects which
are proportional to the full scale output (FSO). Therefore it
is important to select the appropriate pressure range for the
specific application.
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3.1 Temperature Characteristics of Piezoresistive Sensors
The measuring element of a piezoresistive pressure sensor is a
single crystal silicon wafer into which resistors are implanted
in a Wheatstone-Bridge configuration. The properties of the
resistors can be influenced by temperature making compensation necessary. The temperature behaviour is characterised
during manufacture and compensated for using selected
resistors (analogue compensation) or digitally corrected utilizing polynomials. The remaining error can be further reduced
by performing a zero-point correction on the sensor signal.
Fig. 4 shows the sensor signal output with respect to the
applied reference pressure.
(o)
"Zero-pointcorrection"
Sensor signal
(2)
(α)
(3)
(1)
(1) Calibration curve at Tref
(2) Characteristic curve at
TA before "Zero-pointcorrection" (includes
sensitivity and offset
error)
(3) Characteristic curve
after "Zero-pointcorrection" at TA
(α) Thermal sensitivity
error
(o) Thermal offset error
4
Impact of the Sensor Position on the Measured
Pressure
The impact of the sensor position on low pressure measurement and therefore calculations of the gas exchange analysis
have been investigated extensively on a V8 spark ignited
engine. The high and the low pressure measurements have
been obtained from cylinder 4, which has good measurement bore accessibility and a representative pressure curve.
The disturbance from neighbouring cylinders on the same
bank is low, this is due to the angular ignition spacing of
180 °CA. The engine is equipped with variable valve timing
(cam phasors – intake and exhaust), which was used as an
important variable to control the residual gas mass during
this investigation. The piezoresistive (PR) absolute pressure
sensors (described in chapter 2) were both direct mounted
and installed in either a cooling adapter or a cooled switching adapter (Fig. 5 and Fig. 6). The cylinder pressure measurement utilized a water cooled M10-sensor Type 6061B
mounted flush within the combustion chamber on the intake
side. Both the cooling adapters and the cylinder pressure
sensor are cooled by a temperature conditioning unit Kistler
Type 2621.
B, Type 4007B
direct mounted
Remaining error
1 bar
(Ambient pressure)
Fig. 4:
Reference pressure [bar]
C, PR sensor in
cooled switching
adapter
Schematic view of the zero point correction
The calibration reference curve (1) shows the ideal calibrated
characteristic of a sensor and therefore each deviation from
this perfect curve is exhibited as an error. Exposing the sensor
to an arbitrary temperature TA produces both a zero-point
and a sensitivity error (here shown positive) which generates curve (2). The zero-point error and a certain part of the
sensitivity error can be corrected by applying a zero-point
correction to TA at ambient pressure at the time of the test.
Having done the zero-point correction (3) and assuming
that the temperature is stable, the remaining inaccuracy is
caused only by the sensitivity error. The achievable accuracy
of piezoresistive sensors on a test bed and the optimal zeropoint procedure is described in section 4.
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approx.
200 mm
A, Type 4007B
direct mounted
Fig. 5:
Measurement locations in the intake system
7
2
p cylinder
Exhaust,
pos. 2
EVC timing
2,25
IVO timing
4, PR sensor
in cooled
switching
adapter
EVO timing
2, PR sensor in
cooled switching
adapter, frontal
position
2,5
Pressure [bar]
1, Type 4007B direct mounted
in cylinder head, rear position
Exhaust, pos. 3
1,75
Exhaust, pos. 4
1,5
Exhaust, pos. 5
Exhaust, pos. 1
1,25
1
TDC
0,75
270
One of the objectives was to determine the cyclic pressure
fluctuations at the different measurement positions. This
required a correction to the pressure level of the intake
and exhaust pressure curves after the measurement. As a
reference, the pressure curves of the sensors installed in the
cooled switching adapters were used. By using the switching
adapter, the sensor can be referenced to the known ambient
pressure easily and so corrected accurately. The correction of
sensors installed directly or in cooling adapters was applied
during a certain crank angle window when a negligible pressure dynamic existed. Therefore, the absolute pressure level
(given by sensor properties and zero-point adjustment) and
the pressure dynamic (given by the measurement position)
are independent. The following corrections were made:
Intake
• Averaging of whole working cycle
• Reference pressure uses the sensor signal obtained from
the cooled switching adapter, which was referred to the
ambient pressure before each measurement
Exhaust
• Averaging in a crank angle window when a negligible
dynamic pressure exists (0 ... 270 °CA)
• The reference is the pressure measured by a sensor located
in the cooled switching adapter, this in turn, was referred
to the ambient pressure before each measurement
Fig. 7 shows the pressure curve during the gas exchange
at 2 000 1/min and full load. Following the EVO distinctive
differences in the pressure dynamics are visible in the exhaust
manifold (range of 360 °CA).
8
720
Measurement position 2 (frontal position in the exhaust bend,
see Fig. 6) shows distinctive differences in the gas dynamics at
high engine load. This measurement position shows, in each
case, the largest local peak pressure at the beginning of the gas
exchange process. The pressure increase at the frontal measurement position in the bend section of the manifold is caused
by the redirection of the exhaust gas flow. Measurement
position 1 (in cylinder head, exhaust) shows, in each case,
the smallest local pressure maximum during the initial exhaust
blow down. Due to the small cross sectional area high flow
velocities are reached and static pressure fractures are as a
result of this low. Measurement positions 4 and 5 (on straight
duct, distant from valve) show identical pressure curves, even
at high revolutions and high load.
2,25
2
1,75
EVC timing
At each engine operating point 200 single cycles with a resolution of 0,5 °CA were acquired and averaged.
630
Measured pressures in the low pressure phase of the engine cycle at different positions in the exhaust, operating condition 2 000 1/min, full load. Average over 200 single cycles
IVO timing
Measurement locations in the exhaust system
450
540
Crank Angle [deg CA]
EVO timing
Fig. 6:
Fig. 7:
Pressure [bar]
3, PR sensor in
cooled switching
adapter,
rear position
5, PR sensor in
cooling adapter
360
1,5
1,25
Intake,
pos. B
Intake,
pos. A
1
p cylinder
0,75
TDC
0,5
270
360
450
540
630
720
Crank Angle [deg CA]
Fig. 8:
Measured pressures in the low pressure phase of the engine
cycle at different positions in the intake, operating condition
5 000 1/min, full load. Average over 200 single cycles
Focussing on the intake, measurement position A (in cylinder
head, close to valve) exhibits distinctive differences in the gas
dynamic with respect to the engine revolutions, load and valve
overlapping. The propagation of the pressure wave during the
intake stroke moves from the valve back into the intake manifold passing the sensor adjacent to the valve (Position A Fig.
8) then shortly afterwards reaching the more remote sensor
(Position B Fig. 8) with a reduced amplitude. The flow characteristics at position B are due to the configuration of the variable intake manifold. A conclusion would be that the measurement of the pressure at the valve gap is not viable.
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The second sensor is installed directly in the exhaust port in
the cylinder head (Pos. 1), this means that there is no additional cooling device. A maximum temperature of approximately 170 °C is measured at the same operating points, well
below the maximum allowable 200 °C, which means that the
thermal error can be easily compensated.
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10
0
-10
-20
-30
-40
(ho
t)
ad
top
es
gin
En
n –1
, fu
mi
mi
n –1
, fu
ll lo
ad
ll lo
ll lo
00
50
20
00
t)
(ho
top
Fig. 10:
ad
Sensor Type 4045A (M14),
Pos 5
es
One of the two sensors installed in the exhaust is located
in the manifold (Pos. 4). This sensor, mounted in a cooling
adapter reaches a maximum temperature of 80 °C. Using
sensors in cooling adapters leads in general, to temperatures
in the range of 50 … 80 °C across the whole engine operating range.
Sensor Type 4007B (M5),
Pos 1
gin
5 000 min–1,
full load
3 400 min–1,
full load
2 000 min–1,
full load
Measured sensor temperature at intake and exhaust positions
10
0
-10
-20
-30
-40
En
Fig. 9:
2 000 min–1,
part load
Engine
stop (hot)
15
Absolute pressure
error [mbar]
35
34
55
d
75
Pos 1
Pos 5
mi
n –1
, fu
95
180
160
140
120
100
80
60
40
loa
115
00
135
Sensor
temperature [°C]
Sensor temperature [°C]
155
art
175
Intake: sensor in cooling adapter (pos. C)
Intake: sensor direct mounted in cylinder head (pos. A)
Exhaust: sensor in cooling adapter (pos. 4)
Exhaust: sensor direct mounted in cylinder head (pos. 1)
To show the resulting sensor errors which are mostly caused
by thermal effects, an engine load sweep was performed
(Fig. 10). The operating points are chosen in order to successively increase the thermal load into the sensor. All sensors
are first conditioned at 2 000 1/min, part load, and then set
to the ambient pressure.
20
5.1 Sensor Temperature over the Engine Operating Range
A characterisation of the temperature in different sensor
locations was carried out on the V8 gasoline engine. For the
sensor, direct mounted in the intake manifold, this resulted in
temperatures around room temperature. The direct mounted
sensor in the intake port of the cylinder head reached
temperatures of 65 °C (2 000 1/min part load) and 90 °C
(2 000 1/min full load) . The higher temperature level can be
explained by the heat impact of the cylinder head and the
air mass flow. Sensor temperatures at different positions are
shown in Fig. 9.
Having installed the sensors into cooling adapters, the zeropoint correction can be performed accurately, due to well
conditioned sensors and a low dependency of the sensor
temperature on the engine load. When mounting the sensors
directly to the manifold, the sensor temperature may change
according to the engine load (see Fig. 9) and lower the precision of the zero-point correction. This is more noticeable for
the exhaust pressure measurement with the sensor installed
directly in the cylinder head, here significant temperature
changes may be evident.
n –1
,p
To achieve thermal stability of a sensor it is essential that the
sensor is cooled adequately. No sensor is able to achieve the
required accuracy at temperatures sometimes over 1 000 °C,
so cooling the sensor is mandatory. The correct, stable cooling of the exhaust pressure sensor will lead to an almost constant temperature environment for the pressure sensor during
the measurements over the entire engine operating range.
5.2 Accuracy Achieved by Using Piezoresistive Absolute
Pressure Sensors
5.2.1Pressure Measurement with Piezoresistive Sensors
Direct Mounted/in a Cooling Adapter
Whether sensors are direct mounted or in cooling adapters
the measuring element is always exposed to the exhaust
pressure, making it is necessary to stop the engine and reference the sensor to ambient pressure. As presented, a change
in temperature causes a zero-point and a sensitivity error,
therefore, in order to reach the high accuracy required, the
sensor must be in the same condition (mainly temperature),
as it will be during the subsequent measurements, prior to
applying the zero-point correction.
mi
Characterisation of Measuring Accuracy and Influence
on the Analytical Results
High temperatures interacting on a sensor can cause thermal
error which leads to reduced overall accuracy. As the accurate
measurement of the pressure level is the most critical aspect,
a procedure for temperature compensation is necessary to
achieve the high requirements necessary to determine the
pressure level.
00
5
Absolute pressure error with different piezoresistive sensors
mounted in the exhaust in various operating conditions
during a measuring campaign. Reference measurement in
cooled switching adapter in position 4
9
The increased engine load applies a higher thermal load into
the sensor causing an increase of the temperature at the
measuring element. The sensor error is therefore linked to
the applied temperature. The biggest increase in temperature, and hence the largest sensor error, occurs at the sensor
installed in the cylinder head.
In addition to the cited effects of temperature the sensor stability will be evaluated next. Having completed the specified
load conditions over the engine operating range, the engine
is stopped and the difference in the sensor output, between
the first last measuring point is determined. To state the
short term instability, the engine is held at a steady operating
condition and the change in the maximum sensor errors are
obtained.
Mounted in the intake, the environment is less challenging
as both the ambient and media temperatures are significantly
lower than those that surround the exhaust manifold. Sensor
errors (Fig. 11), even of those sensors mounted directly in the
cylinder head, are less than ±0,2 %FSO and stability is very
good also. Less than ±0,05 %FSO difference exists between
the readings taken at the first and last measuring points.
This is due to a combination of factors, the extremes in temperature are less damaging to the sensor and the additional
cooling provided by the charge media help to provide a stable
diaphragm temperature.
INTAKE
Sensor Type 4007BA5FS
Pressure range
0 ... 5 bar
Total error
(typical)
Instability
(typical)
Fig. 11:
Installation with cooling
adapter
±5 mbar/
±0,1 %FSO
Direct installation in cylinder
head
±10 mbar/
±0,2 %FSO
Short-term instability (at
same operating condition)
±2,5 mbar/
±0,05 %FSO
Long-term instability
(between first and last
measuring point)
±2,5 mbar/
±0,05 %FSO
Typical total absolute pressure error and instabilities of low
pressure indication. Sensor type 4007BA5FS installed in the
intake
On the exhaust side (Fig. 12), the sensor errors are greater
due to more dynamic temperature environment, relative to
the intake. The sensors that are installed in cooling adapters
have errors of less than ±0,4 %FSO. The sensor installed
in the cylinder head, because of the elevated temperature
levels, displays the highest errors, up to ±0,9 %FSO. The difference in accuracy therefore, is not dependent on the sensor
type but on the quality and stability of the sensor cooling.
The short term instability for all sensor types and measuring positions, remain within 0,05 %FSO. This characteristic
is especially important when considering using sensors with
cooled switching adapters.
EXHAUST
Sensor Type
4007BA5FS
Sensor Type
4045A5V200S
Sensor Type
4075A10V200S
Pressure range
0 ... 5 bar
0 ... 5 bar
0 ... 10 bar
Installation with
cooling adapter
±20 mbar/
±0,4 %FSO
±20 mbar/
±0,4 %FSO
±30 mbar/
±0,3 %FSO
Direct
installation in
cylinder head
±45 mbar/
±0,9 %FSO
–
–
Short-term
instability (at
same operating
condition)
±2,5 mbar/
±0,05 %FSO
±2,5 mbar/
±0,05 %FSO
±2,5 mbar/
±0,03 %FSO
Long-term
instability
(between first
and last measuring point)
±20 mbar/
±0,4 %FSO
±5 mbar/
±0,1 %FSO
±10 mbar/
±0,1 %FSO
Total
error
(typical)
Instability
(typical)
Fig. 12:
Typical total absolute pressure error and instabilities of low
pressure indication. Three piezoresistive sensors installed in
the exhaust
5.2.2Pressure Measurement with Piezoresistive Sensors
Installed in a Cooled Switching Adapter
A cooled switching adapter has the feature whereby, a pneumatically controlled valve provides switching between ambient and exhaust pressures. The use of a cooled switching
adapter enables a precise and flexible zero-point adjustment
of the piezoresistive pressure sensor referenced to ambient
pressure at any time. The adjustment can be made while the
engine is running under the same thermal load as the following measurement will take place.
In addition, the sensor installed in the cooled switching
adapter, has reduced exposure to extreme conditions like
thermal load and soot contamination.
Using an established measuring procedure in addition to regular use of the cooled switching adapter, as shown in Fig. 13,
high process reliability can be achieved. In each case where
a verification is made prior to every measuring point, even
the smallest thermal zero-point error can be measured and
therefore corrected, ensuring the most accurate scaling.
With this procedure for zero-point adjustment done, a reference accuracy of ±1 mbar can be achieved at every single
operating point. It should be noted that the short term stability during an engine test point is not corrected (Fig. 14).
The difference in the sensor output between the first and last
measuring points can be attributed to the sensor type. Oil
filled sensors (Type 4045A/Type 4075A) show a very small
change, while DCE-Sensor (Type 4007B) exhibits a more
noticeable instability.
10
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Status
cooling
switching
adapter
Ambient
(not switched)
Exhaust
(switched)
Ambient
(not switched)
Exhaust
(switched)
Ambient
(not switched)
Purpose
Protection of
sensor
Warm sensor
(approx. 60 s)
Zero-point
correction
Measurement
Protection of
sensor
Engine
status
Any
Operating point stabilized
Any
Time
Fig. 13:
Procedure for zero point correction of the sensor in the cooling switching adapter
EXHAUST
Sensor Type
4007BA5FS
Sensor Type
4045A5V200S
Sensor Type
4075A10V200S
Pressure range
0 ... 5 bar
0 ... 5 bar
0 ... 10 bar
Total
error
(typical)
Instability
(typical)
Fig. 14:
Installation with
cooling switching
adapter
This error can be eliminated by making a
zero-point correction
Short-term instability (at same
operating condition)
±2,5 mbar/
±0,05
%FSO
Long-term instability (between
first and last
measuring point)
This error can be eliminated by making a
zero-point correction
±2,5 mbar/
±0,05 %FSO
±2,5 mbar/
±0,03 %FSO
Typical total absolute pressure error and instabilities of low
pressure indication. Three piezoresistive sensors installed in a
cooled switching adapter in the exhaust
5.3 Low Pressure Indication with Piezoelectric Sensor
and Pneumatic Pressure Measurement (Remote
Sensing System)
Should low pressure indication be attempted utilizing a
piezoelectric sensor, an additional pressure measurement is
necessary to determine the static mean absolute pressure.
The pressure measurement consists of a pressure tap or
connection point, the connecting hose and a piezoresistive
absolute pressure sensor (Fig. 15). The pressure trace can be
Charge amplifier
HP Filter
Pressure p
PR sensor
Water cooled
PE sensor
Tube
length L
Average
Wall static pressure tap
Fig. 15:
Low pressure indication with piezoelectric sensor and pneumatic pressure measurement for the acquisition of the mean
absolute pressure. The pressure curve results from the addition of the fluctuation with the averaged value
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calculated by the addition of the averaged pressure level and
the pressure oscillation around the mean level. Therefore it is
important that the signal of the piezoelectric sensor has no
static component and therefore a mean value of zero.
The advantage of this method is that a conventional piezoresistive pressure sensor can be used, because of the hose
length the gas temperature at the sensor is low and the
contamination by soot is less likely. The disadvantage is that
with a remote sensing system, as it is installed generally, significant measuring errors can occur with the determination of
the mean pressure level. The error is not related to acoustic
phenomena such as pipe oscillations, but is attributed to the
inflow and outflow of gases through the pressure port.
The pressure oscillation in the intake or exhaust will travel
through the pressure tap and the hose until it reaches the
pressure sensor. Due to this pressure variation within the
remote sensing system, a temporary non-constant mass
flows in and out of the pressure tap. The typical geometries
used for the pressure tap leads to a difference between the
drag coefficient ζI during inflow and drag coefficient ζO during outflow. This leads to a better emptying of the system
compared with the inflow, which in turn results in a reduced
mean pressure level in the remote sensing system.
These effects have been analyzed and demonstrated by
Weyer [6] experimentally as well as through simulation.
Weyer shows that the error during the determination of the
pressure level in a fluctuating pneumatic system is related to
the dimensions of the remote sensing system. Of the most
influence, is the pressure tap itself where the amplitude and
frequency of the pressure oscillation have a major effect. One
exception is, when the pressure tap has the same drag coefficient for inflow as for outflow, this would avoid any error but
it means a complex geometry of the pressure tap and cannot
be accomplished easily.
For evidence about this effect, Eng [7] performed measurements on a single-cylinder diesel engine. He employed a
remote sensing system which was compared to the results of
a direct mounted piezoresistive sensor located in the cooled
switching adapter. The resulting pressure fluctuation and
11
Fig. 16:
12
Pressure curves of direct pressure measurements with
piezoresistive (PR) and with piezoelectric (PE) sensor, pneumatic pressure measurement (tube length 0,3 m) with
PR sensor. Operating condition 5 000 min–1, full load.
Average over 200 single cycles
20
Resulting averaged pressure (0 ... 720 °CA):
Direct PR measurement 1,361 bar
Pneumatic PR measurement 1,336 bar
Fig. 17:
(ho
t)
top
ad
ad
es
TDC
ll lo
720
En
gin
Crank Angle [deg CA]
630
n –1
, fu
TDC
540
mi
450
ll lo
360
50
00
270
n –1
, fu
180
es
90
mi
-0,5
0
top
0,25
-60
ad
-0,25
00
Direct PE
meas. pos. 5
0,5
Position 4
-40
ll lo
0
34
0,25
n –1
, fu
1
0,75
0
-20
d
0,5
mi
0,75
Position 2
-60
loa
1,5
1,25
-40
00
1
20
1,25
Pneumatic PR
meas. pos. 5
En
gin
Absolute pressure in exhaust [bar]
1,75
0
-20
art
Pegged
direct PE
meas. pos. 5
2
1,5
Direct PR
meas. pos. 4
Pressure PE measurement [bar]
2,25
The difference between the absolute pressure measured using
the remote sensing system to that of the direct piezoresistive
sensor installed in a cooled switching adapter is shown in
Fig. 17 for different operating conditions. It can also be seen
that the sensor position has impact due to the differences of
dynamic pressure at different locations.
in –1
,p
It is quite visible, that the remote sensing system indicates
clearly reduced pressure amplitudes as well as a phase shift.
This effect will become more evident towards higher engine
speeds or with a prolongation of the hose.
In the case of low pressure indication with a piezoelectric sensor in combination with a remote sensing system, generally a
systematic error of up to 20 mbar can occur. This correlates
to the error described by Weyer [6]. Therefore this method is
not recommended to achieve the best possible accuracy.
(ho
t)
• Direct pressure measurement with piezoresistive pressure
sensor in cooled switching adapter: Zero-point is adjusted
before the measurement according to the ambient pressure level
• Remote sensing system with piezoresistive sensor (at the
end of the hose): Zero-point is adjusted to the ambient
pressure level during engine stop before the measuring
campaign
• Low pressure indication with a piezoelectric sensor and the
remote sensing system: the pressure oscillation, measured
by the piezoelectric sensor and the averaged pneumatic
pressure are added.
• Direct pressure measurement with piezoresistive pressure
sensor in cooled switching adapter: thermal related sensitivity error (small error, reference measurement)
• Remote sensing system with piezoresistive sensor: Error
related to the pressure tap (inflow and outflow), dependent on hose length. The formation of condensation in the
hose will cause a dampening effect and has an influence
on the dynamics of the signal (considerable error possible)
• Direct pressure measurement with piezoelectric sensor:
thermal related sensitivity error, thermal shock (small error
in the dynamic pressure)
00
m
The following results, measured on the 8-cylinder engine,
include pressure curves from the remote sensing system compared to a piezoresistive sensor installed in a cooled switching
adapter (Fig. 16). The measuring positions 4 and 5 are on
the same longitudinal position in the exhaust manifold. The
measured pressure traces were corrected in the following
manner:
It is evident from the data that the remote sensing system
still shows a pressure dynamic, therefore, averaging the signal
over a complete cycle is mandatory before adding the piezoelectric dynamic component. Compared to the averaged direct
piezoresistive pressure measurement the averaged pressure
of the remote sensing system is too low. The following errors
have an influence:
Absolute pressure error [mbar]
phase shift of the pneumatic signal are strongly related to the
dimensions (diameter and length of the pressure tap, diameter and length of hose and dead volume). It becomes evident therefore, that the error of the remote sensing system,
regarding the determination of the mean pressure level is in
the range of 15 … 20 mbar, which is a rather high number
for this application. A comparison of Weyer's [6] results leads
to a good correlation.
Absolute pressure error of the low pressure indication with
piezoelectric measurement and pneumatic pressure measurement (averaged pressure in window 0 ... 270° CA, see
chapter 3). Direct piezoresistive measurement in position 5 as
reference
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Mass flow rate [g/s]
60
Exhaust mass flow
40
Intake mass flow
20
0
TDC
-20
1,2
Exhaust pressure, pos. 1
1
Pressure [bar]
5.4 Influence of Absolute Pressure Level on the Result of
the Gas Exchange Calculation
Use of low pressure indication for gas exchange optimization
shows that a link exists between the measuring task, the
measuring position, the selection of low and high pressure
instrumentation as well as the analytical process itself. The
technology selection should be done with careful consideration to the boundary conditions as well as the mission targets.
There should be a definition of the quality assurance as well
as a confidence check of the measuring results in an early
stage.
0,8
p cylinder
The following two illustrations show the gas exchange of the
V-8 engine with a fully variable valve train. The engine operating conditions analysed are 2 000 1/min part load restricted
(full valve lift, Fig. 18) and unrestricted (exhaust valve full lift,
intake valve part lift, Fig. 19). Representative pressure curves
are shown, which are measured with piezoresistive absolute
pressure sensors Type 4007B close to the valve at the intake
(position A) and exhaust (position 1), as well as the calculated
mass flow.
Low pressure indication delivers pressure curves in high resolution for all control strategies.
The pressure differences related to the measuring position
at the pre exhaust (Fig. 7), have no impact on the global
results of the gas exchange calculation. This is because of
7.5
Exaust valve lift
0,6
0,4
The gas exchange process is mainly influenced by the pressure difference at the valves. Therefore low pressure and
in-cylinder indication should be considered complementary.
Piezoresistive sensors for intake and exhaust measurements
offer an accuracy in the range of ±10 mbar, however, the
thermal shock error of the piezoelectric cylinder pressure sensor is at least one order higher. The highest uncertainty of the
gas exchange measurement is therefore the in-cylinder low
pressure signal.
Intake pressure, pos. A
0,2
270
360
450
TDC
Intake valve lift
540
630
720
Crank Angle [deg CA]
Fig. 19:
Measured pressure curves during the gas exchange and
computed mass flow rate through the valves. Operating
condition 2 000 min–1, IMEP 2 bar, unrestricted operation,
exhaust valve lift full, intake valve lift reduced. Average over
200 single cycles
the phasing which is considerably before the residual gas
relevance range of the valve overlap. The difference in pressure dynamic at different measuring positions at the intake
(Fig. 8) has just minor effects on the results of the 1-D gas
exchange calculation. The reason for this is that this calculation program takes into account the exact position of the
sensor and therefore the runtime error of the pressure wave
is considered.
An extensive parameter study confirms that primarily, the
pressure level and not the sensor position or their adaptation
is of central importance for the gas exchange calculation.
By increasing valve overlap the sensitivity of the calculated
residual gas fraction on the absolute pressure level in the
intake and exhaust port increases. In Figure 20, an example
is shown on the influence of a different pressure level on the
calculated residual gas fraction.
Intake mass flow
40
20
10
Exhaust mass flow
9
0
8
TDC
-20
1,2
Residual gas fraction [%]
Mass flow rate [g/s]
60
Exhaust pressure, pos. 1
Pressure [bar]
1
p cylinder
0,8
0,4
0,2
270
Intake valve lift
Exaust valve lift
0,6
Intake pressure, pos. A
450
540
7
6
5
4
3
2
1
TDC
360
Valve timings:
Intake -5°CA /Exhaust -5°CA
Intake/ Exhaust series
Intake +5°CA /Exhaust +5°CA
630
0
720
-30
Crank Angle [deg CA]
-20
-10
0
10
20
30
Delta p exhaust [mbar]
Fig. 18:
Measured pressure curves during the gas exchange and
computed mass flow rate through the valves. Operating
condition 2 000 min–1, IMEP 2 bar, restricted operation, full
valve lift. Average over 200 single cycles
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Fig. 20:
Computed residual gas fraction in the indicated cylinder for
variations of the exhaust pressure of ±30 mbar, three valve
timing settings. Operating condition 2 000 min–1, full load
13
6 Conclusion and Recommendations
Miniaturised piezoresistive absolute pressure sensors can be
placed, due to their size and mass, with minimum restrictions
in the manifolds. New high-temperature pressure sensors
broaden the application scope, allowing a sensor installation
even directly in the cylinder head close to the valve. The conditioning of the sensor is still necessary, especially if temperatures are high and sustained as in the exhaust manifold. The
decision as to which piezoresistive sensor and its adaptation
to use has to be considered on a case by case basis.
Measuring Position in the Intake
The choice of the measuring position is easier in the intake
as the temperature of the measuring bore and of the intake
gases normally allow direct mounting of the sensor without
cooling. The use of a cooled switching adapter in addition to
extending the useful life of the sensor, provides a convenient
zero point solution in combustion strategies utilizing high
levels of EGR.
Measuring Position in the Exhaust
Any pressure measurement in the exhaust can be challenging, therefore, when selecting the location for a sensor consideration must be given not only to the physical size of the
adaptation but perhaps more significantly to the geometry
of the manifold. As presented, the dynamic pressure measured at different locations can be influenced by the specific
mounting orientation of the sensor related to the flow.
In the exhaust manifold a cooling adapter will be necessary
unless the sensor can be installed directly in the cylinder head
exhaust runner. Piezoresistive absolute pressure sensors (Type
4045A/Type 4075A) with thin steel isolation diaphragms provide a high resistance to soot emissions and have an acceptable lifetime when constant cooling is present.
Absolute Pressure and Zero Point Correction
Studies on the influence of the absolute pressure on the computed residual gas fraction show that a precision of better
than ±10 mbar is necessary.
The utilization of a cooled switching adapter allows a precise
zero point correction of the piezoresistive pressure sensor,
therefore a reference precision (±1 mbar) can be achieved
in any operating condition. High process reliability can be
assured by using established measuring techniques in conjunction with the switching adapter.
Low pressure indication with a piezoelectric sensor and pneumatic pressure measurement (remote sensing system) is not
recommended for the precise determination of the absolute
pressure level in the exhaust.
Modelling and Simulation
Compared to residual gas models that are based on averaged pressures, a gas exchange analysis referenced to direct
dynamic low pressure measurements provides crank angle
resolved data with a high degree of relevance. A sensor
mounting position near the valve is more likely to provide the
required accuracy for the phasing of pressure at the valve,
which has the added benefit of reducing the demand on the
model.
14
7 References
[ 1 ]
M. Bargende
Homogene Kompressionszündung bei Otto- und Dieselmotoren. Anforderungen und Potentiale
Symposium IAV
Juni 2007 Berlin
[ 2 ]
N. Hoppe
Erfahrung mit dem Einsatz eines modifizierten Restgasmodells und die Weiterentwicklung zum online-fähigen Optimierungstool
Internationales Symposium für Verbrennungs-
diagnostik
Mai 2006, Baden-Baden
[ 3 ]
C. Burkhardt, M. Gnielka, C. Gossweiler, D. Karst, M. Schnepf, J. von Berg, P. Wolfer
Ladungswechseloptimierung durch geeignete
Kombination von Indiziermesstechnik, Analyse und Simulation
9. Tagung,
Der Arbeitsprozess des Verbennungsmotors
September 2003, Graz
[ 4 ]
A. Wimmer, R. Beran, G. Figer, J. Glaser,
P. Prenninger
Möglichkeiten der genauen Messung von Ladungswechseldruckverläufen
Internationales Symposium für Verbrennungsdiagnostik
Mai 2000, Baden-Baden
[ 5 ]
H. Alten
Der Ladungswechsel im Rennmotor
MTZ-Konferenz, Ladungswechsel im Verbrennungsmotor
November 2007, Stuttgart
[ 6 ]
H. Weyer
Bestimmung der zeitlichen Druckmittelwerte in
stark fluktuierender Strömung, insbesondere in Turbomaschinen
Dissertation RWTH Aachen 1973
DFVLR, Forschungsbericht/
Deutsches Zentrum für Luft- und Raumfahrt 1974
[ 7 ]
M. Eng
Untersuchung von Sensoren und Messverfahren zur Niederdruckindizierung
Diplomarbeit Fachhochschule Nordwestschweiz
November 2007
www.kistler.com
Applied Pressure Sensors and Cooling Adapters
Low Pressure Measurement in Intake and Exhaust
T
T
T
L
L
L
L
D
D
D
Technical Data
T
D
Type 4005B…
Type 4007B…
Type 4045A…
Type 4075A…
Measuring range
bar
0 … 5/… 10 1)
0 … 5/… 20
0 … 1/… 2/… 5/… 10 1)
0 … 10 1)
Output signal
(amplifier)
V
mA
0 … 10
4 … 20
0 … 10
4 … 20
0 … 10
4 … 20
0 … 10
4 … 20
Min./Max. temperature
°C
–40/125
–40/200
0/140 3)
0/140 3)
Thermal zero shift
±%FSO
<1 (0 … 125 °C)
<1 (0 … 180 °C)
<0,5 (20 … 120 °C)
<0,5 (20 … 120 °C)
Thermal sensitivity shift
±%
<1 (0 … 125 °C)
<1 (0 … 180 °C)
<1 (20 … 120 °C)
<1 (20 … 120 °C)
Linearity and Hysteresis
±%FSO
<0,2
<0,2
<0,3
<0,3
Dimensions
mm
D/L
6,2/4
6,2/4
12/14
9,5/35
M5x0,5
M5x0,5
M14x1,25
M12x1
Description
Miniature sensor ideal for
measuring pressures in
the intake system. Very
compact dimensions,
versatile, high natural
frequency. Available as
PiezoSmart® sensor or
measuring chain with
amplifier Type 4618A
As for Type 4005B…
High-temperature
design, digital temperature compensation
Oil-filled pressure sensor
with steel diaphragm.
Ideal for measuring
pressures in both the
intake and exhaust
system. Available in
different versions with or
without PiezoSmart®, or
as measuring chain with
amplifier Type 4618A
Oil-filled pressure sensor
with steel diaphragm.
Available in different
versions with or without
PiezoSmart®, or as
measuring chain with
amplifier Type 4618A
Application
• Intake pressure
• Intake pressure
• Exhaust pressure in racing engines
• Intake pressure
• Exhaust pressure
• Exhaust pressure
Recommended
mounting/adapter
• Direct installation in
intake
• Direct installation in
intake or exhaust
(cylinder head)
• Adapter Type 7525A • Adapter Type 7533A
• Direct installation in
intake
• Adapter Type 7511
• Adapter Type 7533A
• Adapter Type 7505
• Adapter Type 7533A
T
1)
2)
other measuring ranges available
depends on measuring range
3)
other temperature ranges available
Cooling Adapters
T
L
T
L
L
T
L
T
Technical Data
Type 7511
Type 7505B
Type 7525A…
Type 7533A…
Recommended sensors
4045A…
4075A… in adapter
4075A…
4005B…/4007B…
4005B…/4007B…/
4045A.../4075A…
Dimensions
L
T
Description
www.kistler.com
mm
12,5
11,8
7
13
G1/2"
M18x1,5
M14x1,25
M14x1,25
Damped adapter for
applications with high
vibration
Compact adapter for
sensor Type 4075A
Compact adapter for
miniature pressure sensors. Damped version
available
Switching adapter to
reference sensor to
ambient pressure
15
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